The heads of two young seals have been mounted and stuck to the sign of a seal sanctuary.
The grim discovery was made by staff at Dingle Wildlife and Seal Sanctuary in Co Kerry.
Ally McMillan, animal manager, said the freshly cut seal heads had been nailed to a piece of wood, which was drilled to the sanctuary's sign. Red paint, which resembled blood, was used to scrawl the words: "RIP Cull."
"It was sickening," said Ms McMillan. "We were very worried because we have two seals here at the moment that are about ready to be released, but it wasn't them."
Gardai in the fishing port town have removed the seal heads and signs as part of their investigation. It is feared they were targeted by people who want the sanctuary to close and believe seals are destroying local fishing waters. Dingle Bay is also famous for its eco tours and cruises to spot seals, whales and the country's most famous dolphin Fungie.
The two dead common harbour seals are believed to be about six months old. Ms McMillan said she is not opposed to the culling of seals when carried out humanely, but added: "For people just to take it into their own hands and club or shoot a seal is sickening."
The cull follows the slaughter of around 60 baby seals in the region in 2004 and the suspicious death earlier this year of seals and dolphins along the Waterford coast. No one has ever been charged.
Irish Wildlife Trust (IWT) chairman Padraic Fogarty said: "Time and again it has been shown that people can indiscriminately poison, shoot and dismember our wildlife safe in the knowledge that the law won't touch them. It is high time that the gardai and our National Parks and Wildlife Service did their jobs in this regard and stop standing idly by as our heritage is dismantled."
The IWT said while it accepts seals have caused damage to fishing gear and lost fish catches, it does not agree that culling the animals is the solution.
Animal rights group Aran has offered a reward of up to 5,000 euro for information leading to the arrest or conviction of those responsible. Spokesman Stephan Wymore said: "Animal abusers are cowards and we're hoping this reward will apprehend those responsible for this most sickening act."